Induction heating apparatus



Aug. 2o, 1946.

H. A. STRICKLAND, JR

INDUCTION HEATING APPARATUS 4v Sheets-Sheet E."

Filed Sept. 2, 1945 Harod .5., Sim' lZandJ:

' INVENTON ATTORNEY AUS- 20, 1946 H.-A. s'rRlcKLAND, JR 2,405,985

INUCTION HEATING APPARATUS I Filed Sept. 2, 1943 4 Sheets-Sheet 3 Harold S'Zrz'clamdJr INVENToR ATTORNEY Fiied sept. 2, 1943 4 Sheets-Sheet 4 I N VEN TOR Tf. y 1m, E d l NIN m Nl w @i tr A y S. @Owjlill n, m2 21H @8 Um E@ a .il m2 uw NC 2 i.

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Patented Aug. 20, 1946 INDUCTION HEATING APPARATUS Harold A. Strickland, Jr., Detroit, Mich., assignor, by mesne assignments, to The Budd Company, Philadelphia, Pa., a corporation of Penn- Sylvania Application September 2, 1943, Serial No. 500,903

16 Claims.

This invention relates to induction heating apparatus. More particularly the invention relates to heating apparatus provided with mechanism adapted for holding workpieces of varying lengths including an extreme length beyond that of the furnace casing.

In induction heating apparatus such for example as described in my co-pending application, Serial No. 384,503, filed March 21, 1941, use is made of an induction heating coil within which a workpiece is inserted, the feed opening of the heating coil being normally closed in the heating cycle by a pivoted hood which functions to initiate the heating operation and also to prevent access for removal of the workpiece while the heating current is effective. This is of importance in order to protect the workmen from possible electrical breakdown and shock and also to prevent removal and re-insertion of the workpiece during the continuance of the heating cycle. A disadvantage of this construction resides in the fact that the furnace cannot operate with a workpiece of such length that a portion thereof extends outwardly beyond the hood thus preventing closing of the hood and the initiation of the heating action.

One of the primary objects of the present invention is to provide a holding or electrical grounding device and closure means for a workpiece in an induction heating coil furnace which is effective to clamp a workpiece of extreme length without interfering with the normal heating function of the apparatus. Another important object is to provide means for clamping a long workpiece in the apparatus which also provides a ground in case the workpiece should be removed during the heating cycle. Still another object is to provide a clamping means for workpieces, which is automatically adjustable to workpieces of different diameter. Still another object is to provide a clamping means for a workpiece which in the case of a short workpiece moves to close the feed opening to prevent removal of or contact with the piece while heating is in progress. An additional object is to provide a clamping or closure means for a heating coil which functions in time sequence in relation to the means for initiating the heating cycle. Additional objects are to provide a holding means for a workpiece in an induction furnace which functions normally in conjunction with the means for initiating the heating cycle but which may function independently thereof without disturbance of the cycle initiating means; to provide means for closing a relay circuit simultaneously with the closure means for the feed opening, but capable of independent movement of said closure means; and to provide simplified construction for the operation of the induction heating apparatus. Other objects will appear on examination of the following description of the invention taken with the accompanying drawings, in which:

Fig. 1 is a side elevation of the upper portion of the heating apparatus showing the furnace casing and associated control apparatus;

Fig. 2 is a front elevational view showing the feed opening closure mechanism, partly in section;

Fig. 3 is a vertical section taken through the control box showing the means for initiating the heating cycle;

. Fig. 4 is a vertical section taken along the lines .4-4 of Fig. 3;

Fig. 5 is a horizontal section taken just below the top casing wall of Figs. 3 and 4 and illus- \trating the control mechanism;

Fig. 6 is a detail showing the relationship of workpiece and holding means at the front open- ?ing or mouth of the furnace;

Fig. '7 is a section along the lines `I1 of Fig. 6, and

Fig. 8 is a wiring diagram of the electric circuit.

Referring to Fig. 1, I have shown in outline an electrical induction heating apparatus having particular adaption to the heating of bars or the like. The construction of the apparatus is in general similar to that shown in my co-pending application above referred to and includes three sections, the numeral I0 being applied to the furnace section proper, the numeral Il to `the section including the control mechanism and the numeral I2 to the section containing condensers and bus bar connections.

The furnace section I0 is provided with a casing I3 and a front end plate UI within which is shown in dash outline the base plate I5 supporting C-brackets I6 the latter in turn supporting the induction heating coil unit I1. The feed opening or mouth I8 formed in a. plate 6 attached to the end plate I4 is in extension of the heating chamber I9 of the heating coil and within this chamber and extending outwardly beyond the feed opening is a workpiece 20. Feed ppening guide ring 2| functions to assist in the support of the workpiece 20 as well asA to aid in guiding the workpiece into the end of the coil, Aas described in my co-pending application Serial No. 506,016, filed October 13, 1943, now Patent No. 2,400,660, dated May 21, 1946.

Means for grounding the workpiece or alternately for preventing removal thereof where the workpiece is of such a length as to be included within the casing, is illustrated particularly in Figure 2 of the drawings. In this figure there is shown in horizontal alignment with thecenter of the workpiece 2i) and on either side of the same, clamping mechanisms 22 and23, each consisting of a cylinder 24 secured by bolts 25 to the outer face of the end plate It. These cylinders are each provided with pistons 26 having projecting piston rods 2, the pistons normally being urged outwardly in relation t the Work-- piece by coil springs 28. The inner ends of the piston rods terminate in lateral yokes 29 having pivot rods 3Q on which are mounted rollers 3| forming the clamping or closure elements for the workpiece. Rollers are required because of axial expansion of the workpiece while heating. In operation, when pressure is led into the cylinder chamber, as by pipe lines 32, the pistons are movedVA inwardly, carrying the holding elements 3| into engagement with the workpiece 20 if extending through the feed opening of the furnace, thepressure of the pistons being such as to maintain they rollers iixedly in position against the workpiece, effectively grounding the same through the all-metal structure of the machine frame. In the case of a workpiece included entirely within the furnace casing, the contact rollers move-into close-position with each other and present an effective barrier against removal or Contact with a workpiece. Cross guide plates 33 are welded to the inner edges of the yokes 29 to prevent-'rotation of the roller suspensions on the otherwise rotatable piston rods, as well as for supplementation of the rollers in coacting as a barrier for the feed opening.

Control apparatus for initiating the movement of the holding or closure means 3| as hereinabove described is shown in detail in aFigures 3 through 6, and is referred to generally by the numeral 35. This unit is enclosed in a casing 36 attached by adjustable screw bolts 5 to the end plate I4 above andY toone side of the feed opening of the workpiece and heating coil and directly above the pressure cylinder 23 as shown in Fig. 2. The casing-36 is box-shaped and is provided on its outer face 3l lwith enlarged spaced openings 38 and 39, one above the other. Within the casing 36 and xed to the end plate I4 is a base plate el) to which at the upper end thereof is xed a threeway valve unit 4| extending horizontally from the base-plate 4i! through the opening 38 in the control casing and terminating-in a push button 52 whereby the valve is operated.

The three-way valve structure is shown more clearly in Fig. 5, the valve comprising a casing i3 having an exhaust port 44, and inlet port 45, and an outlet port 55. The inlet port is coupled to an-inlet pressure conduit l1 and the outlet port, through a conduit 98, to the pipe line 32 associated with the pressure units 22 and 23. Within the casing 43 of the valve adjacent the inlet port 45 is a chamber 49 in which a freely floating valve member 5e is adapted to have axial movement. 'This valve member is formed as a circular plate 5| with axially extending edges 52 forming depressions on either side of the plate 5|, the end of the plate adjacent the base 40 forming a'recess for a coil spring 53 which normally urges the valve member 50 axially against the other side '54 of chamber 49. The other recess ofthe valve member 50 holds a soft rubber plate the spring plat-es and the casings 63 or 54.

Adapted for axial movement also within the casing i3 is the valve rod 56 which is axially hollowed at the end adjacent the valve member 5|) to form a channel 51 extending from the free end of the rod 56 to a point 58 where a lateral duct opens from the rod into a transverse duct 59 formed in a stationary member 50 within which the rod 5t is movable' and communicating with the exhaust 44. A chamber 6| separates the rubber plate of member '50 and the end of the valve rod 56, said chamber communicating with the outlet port 45. Within this chamber also is a coil spring S2 surrounding the end of the valve red for retaining the packing in place as shown in Figurey 5,. In this position fluid under -ressure in the clamping mechanisms 22 and 23 will havefree exitythrough the conduits 32 and chamber 6| and duct 57 to the exhaust port d4, thus relieving the pressure in the cylinders 24 of the holdingunits and permitting movement ofthe holding member away from the workpiece.

In Figures 4 and 5 the adaptation of the valve means is shown for use in connection with the operation of electric switches |24 and |25. These switches are included in the casings 63 and 61|v and utilize spring operating plates S5 and 66. These spring plates, as shown, are fastened by screws to the switch casings and hence are freely movable to operate switch plungers intermediate The free ends of these spring plates underlie the annular flange 5l formed on the inner end of the hollow push button ft2, so that axial movement of the push button causes movement of the spring members and consequent actuation of the switches within the casings 63 and 611. In order to secure this vswitch actuation to some degree independently of the actuation of the three-way valve, the push button is'slidingly mounted on the end of an extension of the valve casing 43, so as to have a limited sliding movement relative tothe valve `casing before engaging the valve rod 5Bwhen-mo'ved .toward the rod. To this end the tubular casingt is screw threaded within the push button head, this casing extending to and having slidable engagement with the outer end Geef. a coupling 'le screw threaded to the Valve casing 43. The inner end of this tube 68 is closed byan inwardly extending annular ange except fora central aperture within which the stem of a vheaded member li placed withinA the tube 68 is movable. A coil spring l2 is positioned within the casing GQ which normally urges the headed member 'Ii into the extreme inward limit of movement with sufcient initial pressure so that the spring is compressed further only after the valve rod ,56 is pressed against its rubber seat 55. Thel protruded stem of the member 1| is normally adjacent but out of contact with the outer end of the valve rodtt. The guiding collar F3 on the Valve rod 56 'holds the end of the valve rod in position and also provides a seat for the inner 'and outer coil springs 'M and 'l5 wherebythe head and the attached valve stem are.`

normally urged to outward position and packing is pressed around'the valve rod 55.

The periphery of the annular ange 6l is inclined toward the axis of the valve in a direction awayfrom the cap of the button to form a cam- 'ber 11 and the extension valve casing 10.

' control casing 36.

11 engages the camming surface of the ilange 61 and is held in this position by means of a coil pivot spring 19. It now appears that when pressure is applied to the push button 42 the initial action will be to forcel the pivotal member 11'l in a predetermined fixed position inwardly of its normal inoperative position. The push button is maintained in this inward position until thepivotal member is forced outwardly to such an extent as to permit the ilange of the push'button to, escape from the detent and move to its normal outer limit. The means for releasing the push button and its movementof the pivotalmember 11 comprises the lever arm80 pivotally mounted on the pivot 18 and extending between themem- On the opposite side of the pivot from the lever 8|), the arm 8|, integral with the lever, extends and Yterminates in a pivot pin 82 to which side plates 83 are attached extending freely downwardly to engage the cross pin 84 at the base thereof, thus forming a square-shaped pendent frame. Within this frame and secured to the base plate 40 is an electro-magnet 3 having an armature 85 movable therein in an approximately vertical direction, the outer end of the armature terminating in the double arch-shaped member 81 having apertures adapted to receive the cross pin 84. Spacing tubes 88 are placed on said cross pin intermediate the member 81 and the depending side` plates S3 to hold the frame in xed relationship to said member. With this construction, on energization of the electro-magnet, the attraction of the member 81 results in an upward movement of the same. The frame is lifted thereby operating the arm 8| and causing the lever 80 to press the pivot member downwardly thereby releasing the push button from its inwardly placed position.

` An alternative manual means for operating the lever 80 is also provided in a second arm 90 fixed to the arm 8| and extending generally downwardly therefrom and toward the front side 31 of the The end of this arm terminates in a Contact plate adjacent the aperture 39 in the lower side of the front control plate 3T. Extending rearwardly from the sides of the opening 39 is guide casing 92 having an inner wall formed with a central aperture. Movable within this aperture is a plunger 93 having a linger plate 94 and an end stop 85 attached tothe plunger external to the inward aperture thereof. A spring 36 is coiled around the plunger within the guide casing 92 between the ringer plate 94 and the inner edge of the plunger receiving aperture. The action of this spring is to normally maintain the nger plate at the outer limit of its Amovement but to permit yieldable inward movement of the finger plate whereby the end or the plunger is moved against the contact plate 3| of the lever 90 and thus moves the lever arm 80 downwardly to cause release of the push button.

On reference to Fig. 5 it will be observed that `the connections from the outlet 46 through the pipe. line 48 terminates in a coupling 91, one end of which is connected to the pipe 98 leading into the clamping pipe line 32 (Fig, 2) and the other end connected into the pipe line 99 having connection to a pressure operated switch |84 shown in Fig. 4. This switch is adapted for actuation Whenever pressure is applied to the holding cylinand lmain switches |I5.

ders 24, so that this switch and the holding mechanisms are directly related and act simultaneously. Similarly on release of the pressure in the holding cylinders the pressure switch is operated to open and break the circuit.

Terminal bar |00 mounted on angle plates |04 is also positioned within the casing and attached through the angle plates to the base plate 40.

The wiring diagram of the heating apparatus is shown in Fig. 8 the power source supplying alternating current being indicated at ||0, and the main circuit lines at and ||2. The circuits may be roughly classified in live sections designated as "Supply, Startingf Load safety, Timer and High frequency and identir-led as in the -drawings as YA, B, C, D and E respectively. The supply section A includes fuses |3, overload disconnection heating elements I4, Included also are the main power line 60 cycle pilot lamp H6, water pressure switches ||1, water pressure pilot lamp l |8, Water heater H9, and water thermostat |20. Main lines and ||2 extend from the supply circuit A into the Starting and load protective circuits B and C, these two sections being mutually dependent for operation. Starting circuit B is based on a pilot contactor relay |2| which on energization is adapted to close the contacts of switches |22 and |23. In addition to the basic elements mentioned, the starting circuit includes the starter switches |24 and |25, pilot lamp |26 and main contactor relay switch |21, the circuit passing from main line ||2 through tap line |28, pilot lamp |26 and relay |2| in parallel, main relay switch |21, pilot contactor relay switch |23 and starter switch |24 in parallel, and starter switch |25 to the load safety section C. Also from starter and pilot relay switches |23 and |24 a branch line leads through pilot relay |22 to a sequence or distributor switch |48, which selects the particular furnace of a bank of furnaces for energization, and thence to the timer circuit section D. v

The basic elements of the load-safety section C are the anti-reset relay |29 and load switch contact elements 8a, 8b and |0|a, |0|b. Antireset relay 29 is adapted on energization to close anti-reset relay switches |3| and |32. y

In addition to the Aenumerated basic elements, there is included in the C section the anti-reset pilot lamp |36, and water temperature and flow switches |40 and |4|. The circuit leads from mainline through water temperature and ow switches |40 and 4|, load switch contact elements 8a and |0|a in parallel, through antireset relay switch |32, junction point |44, to hood switch |25 of the B section. Also from main line ||2 the circuit leads through the anti-reset .relay coil |29 and pilot lamp |36 in parallel to a common junction |42; thence by anti-reset relay switch |3| to a common junction point |43. From point |43 the circuit leads through switch contact elements |0|b and 8b to point |42. Switch contact elements ||b and 8b are normally closed, and switch contact elements 8 a and |0|a normally open. Y The timer current section D is formed principally of two timers of any appropriate commercial make together with related elements. The function of the timer is to eiect a timer limit, subject to adjustment, on the amount of heat received by the workpiece and two timers vare used to insure completion of a heating cycle in case one timer should fail to perform properly. Timer |50 includes the timer solenoid |5I, motor v7 |52 and. magnetic timer lockswitch 1| 53.y Assov kciated with this timer arethe-.timer on-relay` |54 and, timer. onrelay -switch |55. LA-switch larm |56 is movable to..on.positionat switchfcontact element |51 andzto 1oiT.-position.atswitch contact element |56. Similarly in ktimer |60 is included.- the solenoid. |.6|, motor |62, and magneticlockfswitch |65. Associated with this timer |60 are timer on-relay |64, timer on-relay switch "|55 and a switch arm |66imovable to on posi tion Contact element at Y|61 and offposition Icontact element at I 68. The timer-section D also includes the tripping solenoid 85 -for operating the mytripping latch, and pilot lamp |10 for indicating "completion ofthe heating cycle.

' The timer circuit is'traced from the sequence selector switch |40 through parallel lines |11 and |12, including timer solenoids 15| and 16| -respectively, to the main line ||2. To maintain power after opening of sequence switch |48, taps Yare made at |13 and |14 on lines |1| and |12 respectively at points between the sequenceswitch .1|48 and solenoid coils |51 and I5! respectively, these tap lines leading through the timer r switches |53and |63 respectively and :a common -return line |15 to the tap point |44 between B and C section. Line |15 also supplies power to the timer switch arms |56 and |66. The timer on-relay coils |54 and |64 are connected to the main line ||2 and to points intermediate the on timer contacts |51 and |61 and the corresponding motor. The tripping solenoid 85 connects to the main line ||2 and to the o timer contact elements and |68.

`Associated with timer section D is vthe main contactor relay 8| with heat-on pilot lamp |82 connected in parallel therewith. Circuit connections with the timer section are made by extension of timer line |15 from point |11 through load switch 1, furnace panel door switches |19 associated with various side or rear panel-doors,

on-relay timer switches Vand |55 to point on the relay |8|. Also the mainline |-|2 is extended from point |83 through point |15Iand starter switch |84 to the main relay |8|. A -pilot lamp IBS-indicating complete insertion of the workpiece connects the power side of the panel door switches |19 with the main line ||2.

The high-frequency section'y E includes Athe main contactor relay switches and a-rectifying circuit for supplying direct current for operation ofthe high-frequency contactor operating coil. There are two main contactor relay switches |90, |9| and the previously mentioned switch |21.y in

Ythe starter section B, also operated by the relay 'The rectifying circuit is made as a conventional bridge |92 and is comprised in the four branches |93, |94, |95 and |96 with copper oxide or equivalent elements inserted, one in each branch, with such polarity as to produce a direct current potential at points |91 and |90, yas indicated. Al-

ternating current is led to points |99 and 200 on'the rectifier from main line ||2 at |16 and from the C-D circuit |15 at point20l, the re- -sultant tap line including inseries a resistance cut-in combination of a resistance 262 Aand normally closed by-pass switch 203 on the resista-nce, and the main contactor relay switch 19|. The -rectied circuit connects in series the main contactor relay switch and they high-frequency contactor operating coil 1204. Energization of lmagnet coil 204 closes the switch 205 of the highfrequency high-voltage circuit 206 'includingf'the power source 201, capacitor-208 and heating coil l 8 1.' Usually the -powerf sourceprovides current at about 800fvolts and 3000 cycles in the-heating f switches |19.

on, this resulting in closure of 1 pressure switch I1 `and flow switch I4 I. The main switches' 5 are vthenclosed, current flow being indicatedbythe VariousV current flow indicating lamps H6, ||8

and |36. Illumination 'oflamp |36-indicates a closed-circuit through anti-reset relay" |29, by way of Vnormally closed'load switch elements |0|b and A 8b. -.Simultarieously,anti-reset re1ay'swtch`es`l3l and |32'are closed/thus locking the'anti-reset relay-'closing and lconditioning thecircuits for application of voltage on the starter section B and theV timer section D on Vintroduction of the workpiece.

(ill

vinitiation of. aheating cycle.

The workpiece is now inserted in `the'furnace chamber causing the closure-of load switch element |0|a and thereby applying voltage to point |44 and starter section B and timer'section'D. VVBefore the workpiece has passed completely over load switch element I0 a itengages the ejector rod, thus actuating the arm of loadswitch 8. This switch'has'two circuit contact* elements 6a and '5b which are simultaneouslyactuated, normally open switch element 8a serving Aon c10- sure to maintain a voltage on the startingand timing circuits irrespective of the action of load switch |0I. Opening of either orboth 8b-or |0| b shifts the anti-reset relay circuit entirelywthrough anti-reset relay switch '|3I, so that should the power be withdrawn from this relay'permitting opening Vof the switch ISI, it cannot again be closed unless both load switch contact elements 8b and |0|b are closed,'this requiring complete unloading ofthe furnace.

' The workpiece is now moved to the rear end 'of the furnace chamber `closing load switch 1 inthe timer D section. The starter button 42 is now pushedin bringing about closure successively of starterswitches |25,` |24 and |84, the closure of switch' |24 being momentary. On the closure of switches |25 and 24, a closed circuit is at once established through normally closed main contactor relay switchV |21 `and the pilot contactor relay |2| with its associated lamp |26. Thereupon the two pilot contactor relay switchesff|22 .and |23 are closed, switch |23 locking the pilot relay |2|1c1osed since starter switchv |24 is closed only momentarily vduring the hood yclosing movement Also power is applied by way ofswitch' 22 to the sequence switch |48.

The system of circuitsisnow conditioned for This will develop when` the sequence switch |48 which rcontrols the sequence of powerapplication `in the furnacebank `establishes a closed circuitto the timersection D. When this occurs timer solenoids |5| and |'6| arev energized, locking switches |53 and |63 are .closed,.timer switch arms :|56 and |06 `moveto on-position at contact elements y|51 and. |61respectively and motors A|52 and '|62 function to start the timing cycle. Simultaneously on-position-relaysI |54 and l| 64 are energized, closing onrelayswitchesL 55 and |65, and since load switch 1, panel-door switches |19 and starter switchl |84 lare already closed, power isat once supplied from circuitl point |44 between sections B land C lthrough line |15, and from main line ||2tthe The effect of opening switch |21 is to de-energi'ze pilot contacter coil |2| and cause the openin'g of pilot contacter relay switches |22 and |23, thus breaking the circuit through the sequence switch. Additionally, closure of main contacter relayswitch |9`| closes a circuit from point 20| on power line through` the resistance cut-in switch 203 to point |99 on the rectifier |92, thus, applying a voltage at point 200 from main line ||2, creating a direct current voltage at points |91 and |98 which is effective through closed switch |90 to energize the high-frequency contactor coil 204. Thereupon, the switch 205 in the main induction heating coil circuit is closed and power from the source 201 is applied to heating coil |1.

`On completion of the heating cycle either timer switch contact arm |56 or |66 breaks contact at elements |51 or |61 and opens relay switches |55 and |65, thus disconnecting relay |8|` and opening the main circuit and stopping the application of power. In addition, timer switch contact arms |55 and |66 engage contact elements |58 and |68 causing functioning of tripping solenoid 85 to open the clamping bars 21 and star-ter switches and |84. The workpiece may now be removed from the furnace by actuating of the foot pedal and/or application of tongs.

From the description of the structure and of th'e electrical circuit and connections as hereinabove made, the particular utility of the invention may -now be apparent. -Operation of the push button 42 to the latching point opens the valve 4| to permit air under pressure to enter the closure cylinders 24 and at the same time introduce pressure to the switch |84 to make possible energization of the main contacter relay |8| and consequent activation of the power source control relay 204. Switch |25 is closed just prier to the latching of lthe button so that in the latched position this switch remains closed, continued pressure on the button causing overtravel of the flange 61 to close switch |24. When pressure on the button is released it returns part way to the latched position, opening switch |24 whose action is thus termed momentary It is pointed out that these three switches are closed in time v sequence. It is also noted thatthe lfunctioning of switch |84 is fully dependent upon the application of pressure to the holding cylinders 24. However, due to the fact that the holding means involve fluid pressure with its yieldable characteristics it is also apparent that the functioning of switch' |84 is not dependent upon the degree of movement of the workpiece-engaging rollers 3| so that switch |84 functions irrespective of the size of the workpiece inserted in the feed opening. This relationship also permits the heating of a workpiece of such length as to extend outwardly appreciably beyond the rollers 3|, in which position the rollers are spaced from each other by the inter-position of the workpiece. It is pointed out further that in the case of a long workpiece which is being subjected to a heating treatment it may be possible to remove the workpiece before the heating cycle is completed but that, if such' removal is made, the piece is effectively grounded until it is well outside of the machine and the chances of its making electrical contact with'any part of theffurnace with .con-i 'i sequent damage is remote.

The particular structure shown and described for accomplishing the objects of the invention have been found to be efcacious and usable. However, itis apparent that modifications of th'e invention as described may be made to meet special conditions ef use and hence I do not desirel to be limited to the specific illustrated means except in so far as required by the scope of the claims as hereto appended,

What is claimed isi l Y, Y 1. In induction heating apparatus, aheating coil having a feed opening foi-,insertion vof a workpiece, a power source, a circuit including said source and coil, a relay circuit controllingthev functioning of said circuit, including a vrelay switch, closurek means for closing said coil feed opening, and single means for closing said relay switch and closure means, said closure meansbeing adjustable independently of said relay switch. 2. In induction heating apparatus, aheating having a feed opening for insertionof av workpiece, a power source for supplying power to-'said coil, iiuid pressure means laterally movable relative to the coil axis to form a yieldable feed opening closure adjustable to werkpieces of difc, ferent diameter, said means having pressure re-.

lease mechanism and means operably connected with said closure means but capable of movement independently thereof for controlling said fluid pressure means.

3. In induction heating apparatus, a heating coil having a feed opening for insertion of a workpiece, a power source and circuit including a switch for supplying current to said coil, a control circuit including a fluid pressure operated switch and means for closing said power circuit switch, closure means for the coil feed opening, said closure means comprising two opposed, linearly movable closure elements, and uid pressure means for closing said control circuit switch and closure elements.

4. In induction heating apparatus', a heatingv coil having a feed opening for insertion of a workpiece, a power source, a circuit includingrsaid source and coil, a relay circuit controlling thel functioning of said circuit, including a relay switch, a starter switch and circuit operatively connected to said relay circuit to permit energization of the relay circuit and relay switch, closure means for closing said coil feed opening, and single means for closing said starter switch, relay switch and closure means.

5. In induction heating apparatus, a heating coil having a feed opening for insertion of a workpiece, a power source, a circuit including said source and coil, a relay circuit controlling the functioning of said circuit, including a relay switch, a starter switch and circuit operatively connected to said relay circuit to permit energization of the relay circuit and relay switch, closure means for closing said coil feed opening, single means for closing said starter switch, relay switch and closure means, means for establishing a heating cycle in said heating coil on closure of said starting switch, and means for disconnecting said starting switch, relay switch and closure means on completion of said heating cycle.

6. In induction heating apparatus, a heating coil having a feed opening for insertion of a workpiece, a power source, a circuit including said source and coil, starting means for initiating coil a heating'cycle in said circuit, closure means for said coilfeedopening, and single means for energizing successively saidy starting "and closure' means;

7. In induction heating apparatus,- a heating coilhaving `a feed 'opening for insertion of ya workpiece, a power source, a circuit including said sourceiand col-.starting means for initiating a heating cycle" in saidpcir'cuit, closure means for saidcoil feed opening,` single means for activatin'g vsaid starting and closure means, andante-- saidc'oil feedopcning; and 'single means for ac' tivatingv said vstarting and' closure means, said starting means including" a device for restoring saidvsingle means to pre-actuating position.

9.`1nv inductionheating apparatus, a heating coilhaving' a feed opening for insertion of a workpiece, a power source, and'controlicircuit connected to 'said source and coi1.,'andclosu're means for closing said/coil: opening effective only uponenergizationof said control means, said closure `4means' forming'a ground connection to the workpiece.'

10.' In induction" heating apparatus, a heating coil having'a feed opening'for insertion "ofi a workpiece, a power source for'supplying current tosaid'coilfa circutbetweensaid coil and Source, a `relayi switch 4in saidcircuit', a 'control circuit including' ,a starting 'switchan'd .a relay" coil for initiating'a heatinglcycle" in 'said heatingv coil by kclosure of saidrelay switch, a closure means fo"`said"feed opening'and meansincorporated in said 'control"circuit for. 'activating saidl closure meansfonfclosure'ofthestarting switch" and releasing s`aid closure me'ans'at 'the end of the cycle.`

11; 'In' inductionmheating"apparatus,- a heating coil having a feed opening for insertion of 'a workpiece,"'a power source for' supplying" currentrto said coilswitch' means controlling current' sup-4 ply .from..said source,""holdingV means for 'yield`V ably'engaging'a workpiece extending within and without said feed opening, lenergizable' means .for actuating ``said holding means, .and movable mans'forenergization of said energizable means' andclosure of-'saidswitchmeans, saidholding means' automatic'ally ""mo'vingtto" close saidn feed opening only on removal ofthe workpiece during the period of switch closure. y

12. In induction heating apparatus, a heating coil having a feed opening for insertion of a workpiece, a power source forsupplying current to said coil, a control circuit for making said power source eifective in said coil during a heatingY cycle, holding means movablelaterally in rela-V tion to the coil axis into engagement with a work-- piece positioned insaid'coil wtha section there,

ofextending out of the feed openingpenergia` able means for actuatingsaidholding means, andY i movablemeans for energization of saidfenergize able` means .and control circuit,` said holding means being adjustable automatically to workpieces Aof Vdifferent diameters and` operable only during the heating cycle.

13. In induction Iheating apparatus, a heating.

coilhaving a feed opening for insertion of a workpiece,v a power source, la circuit .between rsaid source and coil including a switch, a controlcir` cuit for bringing about closure of said switch, and

closure means-for. said .coil feed opening, said control circuitincludingmeans ffor preventing.`

closure of said sourcecircuit.switch-until. closure ofthe feed opening is under wayn 14. vThe combination-according to claimv13 but including additionally means for preventing release ofthe closure means during a heat cycle.

15. In induction heating apparatus, a heating coil having a feedopening for insertion of a work-` piece, `a power source forr supplyingcurrentto said coil, switchV means f-for establishingfow of current from the source to thecoil, acontrol circuitfor causing closureof saidswitch means, closure means for closing said coil lfeed opening' and energizable meansforA actuating saidclosure means to close rsaid opening, and `single movablev means for effecting energizaton'of said energizable means and control circuit.

16. In induction heating apparatus, a heatingV coil having a feed openingifor insertion of a workpiece, a power sourcel for supplying current to' said coil,f switchameans for establishing ilowof current fromthe source to the coil, a control.

circuit for causing closureof said switch means,v closure means for closing said coil feed opening and energizable means foractuating said closure means to close-said-openingfand single-movable means` for effecting energization ofy said energiZ-I able means .and control circuit, said control circuit being dependentuponsupply of power to the closure means for closure'of the switch means.

HAROLD A., STRICKLAND, J s. 

